This invention relates to video and audio signal assignment systems.
A video assignment system may be described as a switching matrix in which a number of video sources may be switched to a number of video destinations. It resembles a telephone exchange for use with video signals. Video assignment systems are used in television studios in order to allow the large number of routing paths to be achieved.
A conventional known video assignment system accepts an analog video signal from any source and switches it by means of analog switches to the required destination. There is therefore within the assignment system, a large number of switches or cross points. These cross points allow the analog signal to be either connected or disconnected.
A diagrammatic form of a video assignment system using analog techniques is shown in FIG. 1 of the accompanying drawings. The analog video inputs of which there are n in this particular example are applied to one axis of the video cross-point matrix. The video cross-point matrix allows any one of those n inputs to be switched to any one or more of the m video outputs. There is therefore a complete matrix within the video cross-point which contain n multiplied by m cross-points.
The control of the video cross-point matrix is undertaken on the basis of time and other control information. The sources and destinations need to be switched at particular times to correspond to the requirement of the program generation or the usage of the equipment within the video system. Thus at one particular time channel A input will be switched to channel L output and at another time channel A input will be switched to channel K output as an example. There is furthermore certain information which controls the time at which the video signals are switched. This is associated with the nature of the particular video signal being handled. If the equipment is associated purely with television then it is normal to switch the cross-point matrix from one destination to another destination during the vertical interval of the television signal.
FIG. 2 of the accompanying drawings shows a typical diode switch which may be used for an analog video assignment system using conventional techniques. This switch is either conducting or non-conducting depending on whether current I1 is flowing or is not flowing. When current I1 flows diode D1 and D2 are forward biased and diodes D3 and D4 are also forward biased. Provided that the diodes are matched the output is then a replica of the input and has a low resistance. During this condition the switch is in the "on" condition. When I1 is switched off and a reverse bias voltage is applied to the diode bridge the output is isolated from the input via the diode reverse leakage and stray capacity. In this condition the switch is in the "off" condition. In a fully complemented video assignment system with n inputs and m outputs there will be a total of n multiplied by m cross-points of this sort. Associated with each of the cross-points will be a drive current and a method of controlling that current.
The problems which occur in an analog system of the sort described are associated with the leakage which occurs across the cross-point switches. When a large number of these are used in a system, the isolation between the inputs and the outputs becomes very difficult to maintain at the required standard.
The object of this invention is an improvement in the video assignment system. The function of the video assignment system remains unchanged.